Nickel-cadmium batteries are widely used for a variety of electrical power storage applications, notably electrical power storage in spacecraft. The basic arrangement, construction, and chemistry of such batteries are well-known and hence need not be elaborated on in this disclosure. It should be noted at the outset, however, that this invention is useful on both hermetically sealed cells and sealed cells equipped with high pressure relief vents. Accordingly, as the expression is used in this disclosure, "sealed nickel-cadmium cells" is intended to encompass both hermetically sealed cells and sealed cells with a high pressure relief vent.
Suffice it to say that sealed nickel-cadmium batteries of the kind used for spacecraft energy storage, while having many desirable features and advantages for this type of service, have a characteristic which detracts from their usefulness, particularly for future satellite and other future spacecraft applications. This characteristic is the tendency for the precharge of the negative electrode of a sealed nickel-cadmium battery cell to gradually increase with charge-discharge cycling of the cell due to oxidation of the hydrolysis products of the nonwoven nylon cell separators. Thus, it has been shown that these separators are hydrolyzed by the potassium hydroxide electrolyte in the cell, and the by-products of this hydrolysis process are oxidized by the positive electrode. This then results in an increased amount of charge input during an ensuing charge required to reach oxygen gassing from the positive electrode. This results in reduction of uncharged negative electrode active material, and hence growth of negative electrode precharge due to the additional charge required to fully recharge the positive electrode. Continued growth of the negative electrode precharge would result in loss of overcharge protection owing to complete depletion of electrochemically active uncharged negative electrode material when the sealed nickel-cadmium cell is fully charged. Loss of overcharge protection, in turn, could result in catastrophic cell failure due to hydrogen pressure buildup during overcharging of the cell and in high charge voltages which prevent normal recharging. Since spacecraft charge control usually results in termination or reduction of charge at a predetermined voltage limit, the hydrogen evolution and high voltage are usually eliminated. However, due to loss of overcharge protection, the high voltage is reached at a lower state of charge, and the battery fails due to incomplete charging.
Up to the present time, this characteristic of nickel-cadmium battery cells has not presented too serious a problem because of the relatively short design service lives of past and current satellites and other spacecraft in which nickel-cadmium batteries were used. Present geosynchronous orbital satellite systems, for example, are designed to operate for five to ten years. In contrast, future systems are planned for 12 years of service and beyond. The growth of negative electrode precharge caused by battery cycling over the shorter service lives of the past and current satellite systems was generally not a serious problem, or at least a problem which could not be resolved or avoided by state of the art solutions. Over the substantially longer lifetimes of future satellite systems, however, negative electrode precharge growth will be a problem which must be resolved in an effective manner in order to conform nickel-cadmium battery service life to the extended service lives of future satellite systems and the like.
Various techniques have been devised, of course, for increasing the service life of nickel-cadmium batteries in general and, more specifically, for increasing uncharged negative electrode capacity. For example, it is known that uncharged negative electrode capacity may be induced into a cell to reduce the negative electrode precharge by overdischarge of the cell. Such overdischarge, however, results in hydrogen evolution from the positive electrode, and this hydrogen reacts, though slowly, with the uncharged negative electrode to chemically recharge the electrode. Accordingly, if the electrode is overdischarged at a relatively low rate, the electrode precharge reducing action of the overdischarge and the electrode recharging action of the evolved hydrogen counteract one another, and no net increase is uncharged negative electrode capacity is achieved. On the other hand, if the overdischarge is performed at a high rate, as required to achieve an increase in uncharged negative electrode capacity, catastrophic failure can occur due to excessive hydrogen pressure buildup.